<script>
    for (let index = 1; index <=30; index++) {
        
        console.log(`第${index}天`);
    }
    for (let index = 0; index < 3; index++) {
        setTimeout(()=>{
            console.log(index);
        },1)
        
    }
    function fn(n){
        var m = n||10
        console.log(m);
   }
   fn(0)
   fn(100)
   fn(null)
</script>